The emerging field of lead halide perovskite-sensitized triplet-triplet annihilation (TTA) in rubrene shows great promise in upconversion applications. By rapidly transferring single charge carriers instead of bound triplet states, perovskites enable a high triplet population in rubrene, yielding low Ith values. In this contribution, we investigate the role of the triplet population on the upconverted emission. Interestingly, two independent rates of TTA can be observed, as well as a sharp drop in the visible emission intensity over several seconds. This effect can be attributed to the triplet-density-based diffusion length: (i) at low triplet populations slow diffusion-mediated TTA yields singlets far from the interface and (ii) higher triplet populations lead to rapid TTA close to the perovskite/rubrene interface. Because of the proximity of the strongly absorbing perovskite, the singlet states created closer to the interface undergo stronger back-transfer to the perovskite film and therefore appear to exhibit a lower photoluminescence quantum yield.